1. Field of the Invention
The present invention relates to a phase shift mask blank and phase shift mask used for fabricating a semiconductor integrated circuit, and each production method, and in particular, relates to a halftone phase shift mask blank and a halftone phase shift mask wherein an intensity of an exposure wavelength light is attenuated by a phase shift film, and each production method.
2. Related Art
As miniaturization technique of semiconductor devices has been progressed, stabilization of a device fabrication process and measures for finer particles have come to attract attention as important matters. In the ITRS (International Technology Roadmap for Semiconductors) in 2002, it is described that a line space of 0.09 μm will be established by 2004, and accordingly, as to resolving power in a photomask, defect-free and super-resolution are required.
Photomasks are used in a broad range of applications, including the fabrication of semiconductor integrated circuit (IC), large-scale integration (LSI) and very-large-scale integration (VLSI) chips. They are basically constructed by starting with a photomask blank comprising a transparent substrate and a light-shielding film made primarily of chromium thereon and processing the light-shielding film by photolithography using UV radiation or electron beams for thereby forming a desired pattern on the film. The market demand for ever higher levels of integration in semiconductor integrated circuits has led to a rapid reduction in the minimum feature size of photomask patterns. Such miniaturization has been achieved in part by the use of shorter wavelength exposure light.
Although exposure using shorter wavelength light does improve resolution, it has undesirable effects, such as reducing the focal depth, lowering process stability and adversely impacting product yield.
One pattern transfer technique that has been effective for resolving such problems is phase shifting. This involves the use of a phase shift mask as the mask for transferring microscopic circuit patterns.
And, an optical proximity correction (OPC) method has also been utilized in addition to the phase shifting to improve resolving power.
As shown in FIGS. 9A and 9B, a phase shift mask (typically, halftone phase shift mask) is generally composed of a substrate 1 on which a phase shift film 2 has been patterned. The mask has both exposed substrate areas 1a on which there is no phase shift film, and phase shifters 2a that form a pattern region on the mask. The phase shift mask improves the contrast of a transferred image by providing a phase difference of 180 degrees between light passing through the pattern region and light passing through the non-pattern region, and utilizing the destructive interference of light at the boundary regions of the pattern to set the light intensity in the areas of interference to zero. The use of phase shifting also makes it possible to increase the focal depth at the necessary resolution. Hence, compared with a conventional mask having an ordinary light-shielding pattern such as chromium film, the phase shift mask can improve resolution and increase the margin of the exposure process.
For practical purposes, such phase shift masks can be broadly categorized, according to the light-transmitting characteristics of the phase shifter, as either completely transmitting phase shift masks or halftone phase shift masks. Completely transmitting phase shift masks are masks in which the phase shifter has the same light transmittance as the substrate, and which are thus transparent to light at the exposure wavelength. In halftone phase shift masks, the phase shifter has a light transmittance that ranges from about several percent to several tens of percent the transmittance of exposed substrate areas.
FIG. 3 shows the basic structure of a halftone phase shift mask blank, and FIG. 4 shows the basic structure of a halftone phase shift mask. The halftone phase shift mask blank shown in FIG. 3 includes a transparent substrate 1 and a halftone phase shift film 2 formed over the substantially entire surface of the substrate 1. The halftone phase shift mask shown in FIG. 4 is arrived at by patterning the phase shift film 2 of the blank and includes phase shifters 2a, which form the pattern regions of the mask, on the substrate 1 and exposed substrate areas 1a on which there is no phase shift film. Exposure light that has passed through the phase shifter 2a is phase-shifted relative to exposure light that has passed through the exposed substrate area 1a. The transmittance of the phase shifter 2a is selected such that exposure light which has passed through the phase shifter 2a has too low an intensity to exposure the resist on the substrate to which the pattern is being transferred. Accordingly, the phase shifter 2a functions as substantially shield against the exposure light.
Halftone phase shift masks of the above type encompass halftone phase shift masks of the single-layer type which are simple in structure and easy to manufacture. Single-layer halftone phase shift masks which have a phase shifter composed of a molybdenum silicide material such as MoSiO or MoSiON were proposed (see Japanese Patent Application Laid-open (kokai) No. 7-140635, for example).
The important matter is that phase shift mask blanks used in such phase shift masks must satisfy optical characteristics such as a transmittance, a phase difference, a reflectance, and a refractive index at exposure light, and at the same time, they must have durability such as chemical resistance, and few defects.
However, when optical characteristics of the phase shift film used in the above halftone phase shift mask of the single-layer type are set at desired values, the composition of the film is univocally determined. Therefore, it is difficult to produce a phase shift film satisfied with the other characteristics required.
In order to avoid this problem, it has been considered that multiple layers of phase shift films (multilayer phase shift film) including layers for satisfying optical characteristics and layers for satisfying the other characteristics such as chemical resistance are formed. However, an actual composition and a structure of the films possible to satisfy optical characteristics and chemical resistance were not known.
Also, a phase shift film is generally formed by a sputtering method. In the method, the phase shift film is formed with a single target such as a metal silicide made by adjusting the composition of metal and silicon, and mixing and sintering them so as to obtain a desired transmittance in the film. However, since the ratio of metal and silicon in the phase shift film produced by the above method depends on the composition of a target, only a single film corresponding to the target can be formed.
In order to increase the transmittance of the phase shift film in such circumstances, there are proposed a method of increasing an oxygen content in the phase shift film and a method of increasing a silicon content therein.
In the case of the method of increasing an oxygen content in the phase shift film, the oxygen content can be increased by increasing a flow rate of gas containing oxygen flown during forming the film by a sputtering method. However, there is a problem that as the oxygen content in the film is increased, its chemical resistance against a cleaning liquid used for cleaning the film is degraded. There is also a problem that as the oxygen content in the film is increased, the refractive index of the film is decreased, and the film thickness for obtaining a phase difference of 180 degrees becomes thicker.
And, in order to increase a silicon content in the phase shift film, there is a method of increasing the ratio of silicon in a metal silicide target used as a sputtering target. However, this method needs to prepare targets depending on each specification of transmittances of phase shift films. Since metal silicide targets are very expensive, this method is not preferred in consideration of production cost and productivity.
Also, when targets are produced, in a region where a ratio of silicon is high in a target, silicon and metal are not evenly dispersed in the target, so that the dispersion is easily unbalanced. When a phase shift film is formed by a sputtering method with the target of which compositions are not evenly dispersed, abnormal discharge easily occurs, and defects are easily generated in the phase shift film to be formed.
Moreover, when the phase shift mask blank having a multilayer phase shift film as described above is produced, each layer of phase shift films is generally formed on a transparent substrate in order by the sputtering method. In this case, plural different targets for forming each layer of the phase shift films are prepared in a film forming chamber of a sputtering apparatus, and each layer of the phase shift films are formed by discharging respective targets corresponding to each layer.
However, there has been a problem that as the number of layers of phase shift films are increased, defects and particles are easily generated in the phase shift films. Accordingly, there was a problem that in the production of a phase shift mask blank having a multilayer phase shift film, it is difficult to produce a phase shift mask blank having few defects.